1. Field of the Invention
The present invention relates to a method allowing to perform prestack migration of recorded seismic events for imaging a part of an underground zone.
2. Description of the Prior Art
Prestack migration is a conventional method of processing seismic data. The technique generally consists, knowing the value of a wavefield at a known depth, on the surface for example, and a model of the distribution of the wave propagation velocities in the zone, in modelling the propagation of the source field and the retropropagation of the recorded reflection data and in seeking phase coherences between these two modelled fields.
There are three main prestack migration types:
shotpoint migration: the source field is the vibrating state generated by the shotpoint and the reflection data are the response of the subsurface to this source field; PA0 plane wave migration, also called common illumination angle migration: the source field is the plane wave considered and the reflection data are the response of the subsurface to this source field; PA0 offset migration: the source field is the one emitted by a shotpoint and the reflection data are the records obtained by the pickup(s) associated with this shotpoint having the offset considered; in such a migration, migration of the data associated with an offset requires as many wave propagation and retropropagation modellings as there are shotpoints and stacking of the results obtained for each shotpoint. PA0 Claerbout, J. F., 1985; Imaging the Earth's interior; Blackwell Publications, PA0 Duquet, B., 1996; Amelioration de l'Imagerie Sismique de Structures Geologiques Complexes; these, Universite Paris 13, or PA0 Whitmore, N. D., Felinsky, W. F., Murphy, G. E. and Gray, S. H., 1993; The Application of Common Offset and Common Angle Pre-stack Depth Migration in the North sea, 55.sup.th Mtg., EAGE, Expanded abstract.
Examples of implementation of this type of techniques are for example described in:
The main drawback of conventional implementations based on the Kirchhoff equation (or more elaborate versions of this technique, itself based on high-frequency asymptotic techniques) is that they are generally very costly in calculation time because of the volume of the data to be processed and of the results, especially when the velocity field varies laterally (which complicates the arrival time calculations required for implementing this method). For economy reasons, one is often led to limit the volumes of data (by decimation) and/or the amount of results produced (imaged volume of reduced size, rough sampling, of the results).